1. Field of the Invention
The present invention relates to paper or sheet product converting machines of the rotary type wherein a serial multiplicity of cutting, scoring, folding and gluing operations are performed on sheet material such as paper drawn from a reel of indefinite length to produce a finished product.
More particularly, the subject invention relates to an improvement in gum box apparatus used as a subcombination element in a rotary converting machine.
2. Description of the Prior Art
The generally practiced prior art technique of applying gum adhesive to precisely designated areas of a paper sheet product blank in transit through a rotary converting machine is to print the gum onto the blank by a rotary image transfer means called a picker as the blank passes between the picker carrying roll and a backing roll. U.S. Pat. No. 2,568,629 to V. E. Heywood is representative of the prior art as presently practiced with a rotary envelope machine.
Relative to the Heywood disclosure, gum adhesive is applied to the picker print face from the surface of a rotating metering roll in the same manner as practiced by the rotogravure arts. A portion of the rotating meter roll periphery is immersed in a viscous, fluidized gum bath. A doctor blade skims the film adhering to the meter roll surface to leave a thinner film of gum having a precisely graduated thickness remaining on the roll surface. Continued rotation of the meter roll past the doctor blade brings the doctored gum film into contact with the print face of the picker which, by viscous fluid adhesion, picks that portion of the film off the meter roll surface onto the picker print face.
As the picker roll continues rotation past tangency with the meter roll and into tangency with a backing roll, a register aligned envelope blank is drawn into the nip between the picker and backing roll where gum on the picker print face transfers to the envelope surface at the desired location.
At the current state of art development, envelope machines such as Haywood's are capable of producing up to 72,000 envelopes per hour and consume 0.5 gallon per hour of adhesive from a single gum box in the process. Such a fluid consumption rate would normally suggest the utility of an automatically controlled, pumped supply of adhesive to the gum box.
Although the concept of pumped gum box replenishment, may, under the circumstances, appear obvious, as a practical fact this has not occurred. Gum boxes on high speed rotary converting machines such as Heywood's are predominantly replenished from small, one liter sized, manually changed vessels which dispense their contents by means of a simple, vacuum regulated liquid level control apparatus.
The operative reasons for continued use of such an archiac gum supply system reside in the physical characteristics of the gum and its rheology. For example, the gum solids are dissolved in a highly volatile solvent to permit rapid drying after application to the envelope blank. However, the gum pond through which the meter roll rotates is atmospherically open thereby permitting solvent loss to the atmosphere prior to application on the envelope. The aforedescribed small gum containers represent an effort to minimize the surface area of fluid gum exposed by the gum pond. Although it is desirable to minimize the pond size, the absolute scale required for manual manipulation of a gum supply container necessitates an undesirably large pond.
As a consequence of the gum rheology, the continuous shear of the meter roll surface through the pond tends to upset and thicken the gum viscosity. The adhesive meter roll has a 1:1 rotational ratio with the article production rate; e.g. an envelope production rate of 1200 envelopes per minute requires 1200 meter roll revolutions per minute. Simultaneously, the adhesive viscosity may, representatively, be in the order of 3000 centipoises at 70.degree. F. The meter roll surface draws the thick adhesive fluid to a shear line at the doctor blade which allows a small portion of the adhesive propelled by the roll surface to pass through the blade gap opening. The remaining portion of the moving fluid inertially drives across the pond bottom to be cyclically returned to the meter roll surface. Resultantly, the adhesive pond is, dynamically, in a continuous, rolling circulation and boundary layer shearing. It is the high frequency shearing of the adhesive that upsets the physical rheology of the substance to further increase the viscosity. Consequently, within the rolling circulation of the adhesive pond, flow channels are developed between a single, centrally located, adhesive supply point and the meter roll surface. Such flow channels are bounded by flow stagnation zones wherein the gum eventually gels. The gelatinization is progressive and finally obstructs all flow regions between the source container and the meter roll surface. When this occurs, the machine production must be interrupted while the gum box is purged of gelatinized gum and thoroughly cleaned.
This latter consequence of gum distribution to the meter roll surface occurs regardless of the gum supply technique; whether by bottle or by pumped replenishment. Since the machine downtime for gum box cleaning represents the major value of operating maintenance losses, there has been little incentive to replace the bottle gum supply system with a pumped system.
It is, therefore, an object of the present invention to teach a rotary converting machine gum box system having mechanical gum distribution means between a gum supply point and the respective meter roll surface.
Another object of the present invention is to provide a rotary converting machine gum box with a rapidly circulating pumped replenishment system requiring minimal atmospherically exposed area and infrequent maintenance cleaning.